Methane Emissions and Microbial Communities as Influenced by Dual Cropping of Azolla along with Early Rice

Abstract

Azolla caroliniana Willd. is widely used as a green manure accompanying rice, but its ecological importance remains unclear, except for its ability to fix nitrogen in association with cyanobacteria. To investigate the impacts of Azolla cultivation on methane emissions and environmental variables in paddy fields, we performed this study on the plain of Dongting Lake, China, in 2014. The results showed that the dual cropping of Azolla significantly suppressed the methane emissions from paddies, likely due to the increase in redox potential in the root region and dissolved oxygen concentration at the soil-water interface. Furthermore, the floodwater pH decreased in association with Azolla cultivation, which is also a factor significantly correlated with the decrease in methane emissions. An increase in methanotrophic bacteria population (pmoA gene copies) and a reduction in methanogenic archaea (16S rRNA gene copies) were observed in association with Azolla growth. During rice cultivation period, dual cropping of Azolla also intensified increasing trend of 1/Simpson of methanogens and significantly decreased species richness (Chao 1) and species diversity (1/Simpson, 1/D) of methanotrophs. These results clearly demonstrate the suppression of CH4 emissions by culturing Azolla and show the environmental and microbial responses in paddy soil under Azolla cultivation.

Azolla cover significantly decreased CH4but not N2O emissions from flooding rice paddy to atmosphere

“…Azolla cover decreased the seasonal CH 4 emission by 34.7% compared to the control treatment without azolla cover (Table 2). This result was consistent with previous recent studies (Bharati et al 2000;Ma et al 2012;Ali et al 2015;Liu et al 2017).The cumulated CH 4 emission from control treatment pot at 55.4 g C m −2 was similar to the value gathered in a field experiments in Yamagata (Itoh et al 2011). Not only azolla, but also common duckweed (Lemna minor), the other floating macrophyte, significantly reduced the mean CH 4 emission from flooded rice paddies in China by 20.4% (Wang et al 2015).…”

“…5) between control and azolla cover treatments, implying that azolla cover did not affect the CH 4 production in the submerged dual cropping azolla-rice soil ecosystem. This confirmed that the decreased CH 4 emission by azolla cover was due to stimulated CH 4 oxidation (Bharati et al 2000;Ali et al 2015;Liu et al 2017). The sources of CH 4 production are from both older matter (e.g., native soil organic matter, incorporated organic material such as straw and manure , and new matter from plant growth (e.g., root exudates and plant debris).…”

“…Chen et al (1997) and Ying et al (2000) reported that azolla application increased CH 4 emission, likely due to the exudation of azolla root and decomposition of dead azolla, but not due to azolla transportation of CH 4 in soil as rice plant did. However, the most recent reports showed that azolla cover decreased CH 4 emission from rice soil ecosystems, likely due to the increase in redox potential in the root region and dissolved oxygen concentration at the soil-water interface (Ali et al 2015;Singh and Strong 2016;Liu et al 2017). In this study, we had not obviously observed the dead azolla until harvest, and even if the exudation of azolla roots contributed to the organic matter in soil surface, the contribution could have become an insignificant source of CH 4 since the surface layer of submerged rice paddy always maintained oxic conditions.…”

Growing Azolla to produce sustainable protein feed: the effect of differing species and CO2 concentrations on biomass productivity and chemical composition

“…As a result of their high growth and N 2 fixation rates, Azolla ferns have been used as a bio‐fertilizer in rice paddies in south‐east Asia for several centuries . Using Azolla as bio‐fertilizer increases the sustainability of rice cultivation by reducing fertilizer requirement as well as reducing nitrogen and greenhouse gas (GHG) emissions from rice paddies . However, a direct use of Azolla biomass for feed production may provide more environmental benefits as well as economic value to farmers, but is not yet common on a large scale.…”

An Exploration of Common Greenhouse Gas Emissions by the Cyanobiont of the Azolla–Nostoc Symbiosis and Clues as to Nod Factors in Cyanobacteria

“…Still, the cultivation of Azolla spp. in methane-emanating rice paddies, which are hot spots for methanogenic phenomena, has been shown to alleviate methane emissions in dual cropping systems [27]. The supposed explanation of this observation is that the dissolved oxygen in irrigation systems and the soil redox potential become favorable under Azolla cultivation [27].…”